posteriority has 2 distinct synonyms spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are anteriority, priority.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the quality of being toward the back or toward the rear end
following in time
Synonyms, antonyms and the broader and narrower relations for posteriority come from Princeton University’s WordNet. Full sources and licences.